The invention relates to a highly reliable optical data network.
In future data networks, such as the digital telephone network for example, optical waveguides with the corresponding transmission equipment will be employed instead of the metal lines used hitherto. An optical data network of this type is described in British Telecom Technology Journal, Vol. 7, No. 2, April 1989, pp. 100-113. The data is transmitted in a passive optical network (fiber to the home/curb) which physically corresponds to a tree network and is logically described by a star network. For converting the electrical signals into optical values, in addition to the electrooptical transducers, control devices are required for performing the transmission line procedures and formatting the data. These are essentially accommodated in a central unit or headend which is installed on the switching side and also handles the error monitoring. The branching in the tree network is handled by optical splitters. Data is always transmitted between the subscriber line units and a central unit, via a switching device, and also between a central unit and the subscriber line units. A time-division multiplexing method (TDM) is employed in the direction toward the subscriber and an adaptive time-division multiplexing method (TDMA--time division multiple access) is employed in the direction toward the central unit. The transmission frames have the same basic structure in both directions. In addition, the data rate for each subscriber is the same in both directions. In order for it to be possible to transmit the data packets from the subscriber line units to the central unit, first of all the different delays resulting from the different lengths of the optical waveguides between the central unit and the subscriber line units must be equalized.
This is performed by electronic delay devices in the subscriber line units, by means of which the total of the delays in the optical waveguides and the transmission equipment are set to the same value for all subscriber line units. The calibration and setting of the electronic delay devices is performed by the control device of the central unit (operation and maintenance--O&M) in cooperation with the individual subscriber line units. The control device performs tasks such as the calibration and checking independently. Other tasks, such as a two-sided configuration of data channels with different transfer rates between the central unit and the subscribers, on the other hand, are only performed when requested by the exchange. Besides the data channels, for this purpose there are additional control and monitoring channels, termed O&M channels herein, between the central unit and the subscriber line units. The central unit always handles the control and monitoring.
The central unit is thus an indispensable component of the overall transmission equipment. Its failure would interrupt the connection to a comparatively large group of subscribers.